A rack is often defined as a standardized frame or enclosure for mounting multiple electronic equipment modules, such as network equipment, computer servers, audio/video equipment and/or the like. Racks typically come in one of two sizes: 19-inch or 23-inch. This size refers to the width of the equipment that can be mounted inside the rack. Differently-sized racks are also possible.
A rack unit “U” (sometimes also referred to as “RU”) is a unit of measurement defining the height of equipment intended for mounting in a rack. One rack unit, as accepted today, is 1.75 inches (44.45 mm) high. Rack equipment is usually manufactured with a height in multiples of U, such as “1U”, “2U” and so on. A typical full-size rack measures 44U, while a typical “half-height” rack, or “half-rack”, would usually have a height of between 18U and 24U. The term “half-rack”, however, may be used differently in different situations. For example, “half-rack” may refer to equipment width; a “half-rack” equipment module may have a width of 9.5 inches (half of 19 inches) or 11.5 inches (half of 23 inches). In other situations, “half-rack” may refer to equipment depth, namely—two “half-rack” modules may be mounted one behind the other.
Formal standards for the more common 19-inch (482.6 mm) rack include the following, which are incorporated herein by reference in their entirety:                Electronic Industries Alliance EIA-310-D, Cabinets, Racks, Panels, and Associated Equipment, dated September 1992. (Latest Standard Now REV E 1996).        Consumer Electronics Association CEA-310-E design requirements for Cabinets, Panels, Racks and Subracks, dated Dec. 14, 2005.        International Electrotechnical Commission—IEC 60297 Mechanical structures for electronic equipment.        